The best days of life in the universe may be behind us

The factors that make a star a suitable host for life on the surrounding planets are complex, and new research shows that something we used to think of as an advantage—a high metal content—may instead be a detriment. If so, it shows that we have focused our search for life-supporting planets in the wrong places. More worryingly, this means that the prospects for life escaping the oceans may be in steady decline.

The early universe consisted of only three elements: lots of hydrogen, some helium, and some lithium. The first stars were made entirely of them, producing other light elements while they lived and heavier ones as they died. These deaths spread these elements, known to astronomers as metals, even if chemists wouldn’t call them that, into the galaxies to be incorporated into future generations of stars.

Metal abundance is therefore considered one of the most important stellar characteristics, and for this reason the search is being made for surviving low-metal stars from the early universe. Yet according to a new study, there’s another reason to look for (somewhat) metal-poor stars: their planets are more likely to have the necessary shielding that could allow life to flourish.

Very metal-poor stars have always been considered poor prospects for life. If a star formed without metals, its planets likely formed as well, allowing only gas giants and not rocky worlds like Earth. More metals (which to astronomers include oxygen, carbon and silicon) means more chances for a world like the one we call home. Therefore, while metal-poor stars are valued for space archaeology, metal-rich ones are prioritized in the search for life.

While the work offers a hint of where efforts to find habitable planets should focus, it comes with a side of existential dread.

Yet Dr. Anna Shapiro of the Max Planck Institute for Solar System Research and co-authors argue that this misses a crucial point. Life on Earth depends on a thin layer of ozone that prevents the Earth from being sterilized by ultraviolet light.

“We wanted to understand what properties a star must have in order for its planets to form a protective ozone layer,” Shapiro said in a statement. The issue was deemed particularly important after a previous study found that the brightness of many Sun-like stars varies more than the Sun, which would make UV an even greater potential threat.

Metal-poor stars emit more ultraviolet radiation than their metal-rich counterparts, the authors found. This might lead one to assume that it is less likely to have life around it, but there is a catch. Ultraviolet light covers a wide range of wavelengths, and ozone is formed in our atmosphere by UV-C light and destroyed by UV-B. The ratio is crucial and is much more favorable in metal-poor stars.

Metal-poor stars produce more UV radiation, but in a form that promotes ozone formation and therefore protects against dangerous wavelengths. Image credit: MPS/

“Contrary to expectations, metal-poor stars should provide more favorable conditions for the emergence of life,” concludes Shapiro. It’s the opposite of the Midas touch. Life may start in the oceans, which act as its own UV shield, on stars richer in metals than the Sun, but its establishment on land may be too far away.

While the work offers a hint of where efforts to find habitable planets should focus, it comes with a side of existential dread.

One explanation for Fermi’s paradox is that, until recently, the only stars in the galaxy rich enough in metals to have suitable planets were near the center. Here, interactions between stars are so frequent that planets can be disrupted. In the safer regions where we are based, perhaps the Sun is simply one of the first stars rich enough in metals to have a suitable planet.

This work, if it stands up to reanalysis, turns that on its head and suggests that it may become harder and harder to develop life as more planets are sterilized. If humanity destroys itself (perhaps by tampering with that precious ozone layer), the chances of a reboot may disappear.

The study was published in Nature Communications.

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